This invention relates to a pick up means or reader for optically reading information recorded on the track of a rotary disk.
It is known to store information, such as video information, on a rotary disk by means of concentrically or spirally arranging minute reflecting dots which are varied in shape and interval in correspondence to a signal such as a television video signal. To read the information thus stored, the video disk is rotated at a constant speed, a light beam is directed onto the track of the video disk, and the reflections,which are modulated with the variation of the dots, are detected and translated into an electrical signal.
A conventional apparatus as shown in FIG. 1 comprises a disk 7, motor 8 and pick-up arm 10. A light beam emitted from a light source 1 (for instance, a helium neon laser) in the pick-up arm is enlarged to have a suitable diameter by a collimator 2 and is focussed on the reflecting surface of rotary disk 7 by means of a beam splitter 3, a quarter wavelength plate 4 for changing the polarization direction, a movable mirror 5, and a focussing lens 6. The quarter wavelength plate 4 is employed for improving the separation of the incident light beam from the reflected light beam, but its presence is not essential. The reflecting surface of the rotary disk 7 is positioned very close to the focal point on the object side of focussing lens 6. Therefore the light beam, as it strikes the disk, is focussed into a minute spot having a diameter of the order of 1 .mu.m. Since information dots on the reflecting surface of the rotary disk 7 are formed at a pitch of the order of 2 .mu.m and have a diameter on the order of 1 .mu.m, the light spot is small enough to discriminate the recorded information.
The reflected, modulated light beam is picked up by the focussing lens 6, and delivered via movable mirror 5 and quarter wavelength plate 4 to the beam splitter 3, where it is reflected toward a light detector 9. The light detector 9 operates to convert the reflected light beam into an electrical signal corresponding to the recorded information. The rotary disk 7 is rotated, for instance, at a speed of 1800 rpm by the electric motor 8, while in order to continuously read the signal arranged spirally or concentrically the pick-up arm 10 is radially shifted as the rotary disk 7 is rotated.
In general, an incident light beam or a reflected light beam is a pencil of light rays whose intensity distribution is such that the intensity decreases with distance from the optical axis as shown in FIG. 2. Therefore the propagation path of a light beam can be represented by the center line maximal in intensity.
In the reader described above, if there is no eccentricity in the rotary disk 7 or the information tracks thereon due to imperfect connection to the shaft of the motor 8 or the like, as the rotary disk 7 rotates, the focussed light spot would follow perfectly the spiral track, and the reflected light beam would accurately reach the light detector 9. However, such ideal conditions are not common and therefore a feedback means which rocks the movable mirror 5 to laterally shift the focussed spot, is employed. This is shown in FIG. 3 where the focussed spot hits the disc 7 to the left of the originally focussed spot (designated x) due to the slight clockwise rotation of mirror 5 by feedback means known in the art. However, there is still the disadvantage that, in order to allow the focussed light spot to follow the spiral track, the reflected light beam is not accurately led to the light detector 9.
If the rotary disk 7 is eccentrically connected to the shaft of the electric motor 8, the spiral track on the disk will be moved right and left, relative to point x, for every rotation of the disk. Since the rotary disk 7 is rotated at a high speed of 1800 rpm, it is impossible to move the whole pick-up arm 10 right and left at such a high speed so as to enable it to follow the track. Accordingly, there appears no alternative to rocking the movable mirror 5, which is small and light, in response to feedback controls to let the light spot follow the track. The arrows in FIG. 3 indicate the incident light beam and the reflected light beam in the case when the mirror 5 is turned with respect to the deflection of the track. As is apparent from ths figure, the incident light beam enters the focussing lens 6 at an angle with the optical axis of the focussing lens 6. Therefore it is focussed accurately onto the track, at a position shifted radially from that in FIG. 1. However, the unfortunate side effect is that the incident light beam is not perpendicular to the reflecting surface of the rotary disk 7, and consequently the reflected light beam therefore advances along an optical path different from that of the incident light beam. As a result the prior art system above has the following disadvantages.
(1) The amplitude of an electrical signal read in synchronization with the eccentricity of the rotary disk is greatly varied, as a result of which the S/N ratio becomes low, and the read signal fluctuates.
(2) The feedback means requires a light detector having a large area light receiving surface, and therefore the S/N ratio and the frequency characteristic becomes low.
(3) The amount of eccentricity allowable for the connection of the rotary disk is considerably small, and accordingly the rotary disk itself and the rotating bearing mechanism must be high is accuracy.
(4) It is known that time axis variations or errors of the read signal due to irregular rotation of an electric motor can be corrected by moving the incident light beam spot forward or backward on the circular or spiral track with respect to the rotating direction of the rotary disk. However, in such correction of the time axis variation, the position of the reflected light beam to the light detector is varied by the turning of the movable mirror, that is, it is impossible to correct the time axis variation with the apparatus of FIG. 3.